Existing digital video recorder (DVR) systems can only process two digital-video data streams at once. Typically, such systems handle “one in, one out”, that is, one data stream coming into the system from a digital tuner, or from an analog tuner with an MPEG encoder, and being written to disk, and one data stream going out of the system, which is being read from a hard drive (HD) disk, fed through an MPEG decoder and NTSC encoder, and displayed on a TV set.
Due to the presence of multiple analog or digital tuners, multiple TV-set display capability, and streaming-media downloading via the Internet, new generation DVRs are required to handle multiple digital data streams simultaneously. Some of these digital data streams are time-critical. For example, a digital data stream coming into a recording system must be recorded accurately to a disk without loss or corruption. Otherwise, a permanent glitch will appear any time the recorded program is replayed. Similarly, a digital data stream being read from the disk and decoded for display on a TV must be delivered in a timely manner. Otherwise, the MPEG decoder will starve and a visible glitch will appear on the screen.
The amount of data flowing through a single digital data stream may vary widely. Limited-quality video may require only 1–2 megabits per second, while high-quality video full of rapid motion may require between 10 and 15 megabits per second. New generation television systems may deliver data streams of 19 megabits per second or more. Moving this much data to and from a hard drive is a challenge.
Inexpensive high-value IDE HD disks today are capable of sustained data transfer rates of anywhere from 6 to 18 megabytes per second. Transfer rates vary according to the disk model, and also to the location on the disk to which the data is being transferred.
An HD disk's outer tracks have a higher linear data density (more sectors per track) than the inner tracks. Because an HD disk rotates at a constant rate, the outer tracks can thus accept and deliver more data per second than the inner tracks. A typical HD disk may transfer data at a rate of 10 megabyte per second on the inner tracks, and a rate up to 18 megabytes per second on the outer tracks.
Transfer rates show only one aspect of the disk's performance. They do not reflect the disk's need to seek between different areas of the disk. An operation typically requires 10–25 milliseconds per seek, depending on distance. In addition, they do not reflect the drive's occasional need to read a data sector more than once due to errors on the disk or to the effect of impact and vibration.
Finally, a DVR's HD disk is often used for more than simply storing data streams of video content. The disk may also hold a file system or database containing information about upcoming programs, on-screen guide, and program scheduling. It may also hold executable software used by the DVR. There may thus be many different clients requiring access to the disk. Some accesses such as digital video write and read streams are critically time-sensitive. Some others are less sensitive to delay. For example, slowing down access to executable code or to a program-guide database may cause the DVR's user interface to behave in a sluggish fashion, but does not cause errors or failure of the system. Some others are not at all time-critical and can be deferred for a significant amount of time without anyone noticing. For example, a background task looking for shows that might be of interest to a viewer can be deferred.
What is desired is a mechanism that would enable a digital recording system to dynamically mitigate competing bandwidth requirements when multiple data streams are simultaneously handled.